The present invention relates to a copier, printer, facsimile apparatus or similar electrophotographic image forming apparatus.
Generally, an image forming apparatus includes a photoconductive element, an exposing unit, a developing unit, an image transferring unit, a fixing unit, a cleaning unit, a sheet feeder, a registration roller pair, an outlet roller, and an outlet sensor. The apparatus additionally includes a heat discharge fan for discharging heat generated in the apparatus, e.g., heat generated by the fixing unit.
Assume that the image forming apparatus forms an image with high resolution based on a low linear velocity, and that the heat discharge fan continuously rotates from the time when a sheet starts being driven by the registration roller pair to the time when it moves away from the outlet sensor. Then, when the leading edge of the sheet passes the outlet sensor, a stream of air generated by the heat discharge fan is apt to cause the leading edge to shake and therefore crease, degrading image quality. A thin sheet, in particular, cannot smoothly enter the fixing unit due to the influence of the stream of air and therefore creases.
Japanese Patent Laid-Open Publication No. 7-319370, for example, teaches countermeasures against the creasing of a sheet ascribable to the heat discharge fan. In accordance with this document, when use is made of a thin sheet, a heat discharge fan is rotated at a lower speed than usual. At the same time, a control member adjoining the heat discharge fan for controlling airflow is replaced with one having a smaller air passage in order to reduce airflow. Further, a motor drives the control member in order to vary its angular position, thereby reducing the airflow of the heat discharge fan.
There is an increasing demand for an image forming apparatus capable of printing images with a plurality of different resolutions and thereby enhancing image quality. An image forming apparatus with this capability may be constructed to feed, for resolution as low as 600 dpi (dots per inch), a sheet at a usual linear velocity or feed, for resolution as high as 1,200 dpi, a sheet by reducing the usual linear velocity to one half.
Laid-Open Publication No. 7-319370 mentioned earlier does not control the rotation speed of the heat discharge fan in accordance with resolution. As a result, when a sheet is fed at the lower linear velocity for the resolution of 1,200 dpi, the stream of air generated by the fan is apt to cause the leading edge of the sheet to shake and crease. We have proposed an implementation for protecting a sheet fed at the lower linear velocity or a thin sheet from creases ascribable to the heat discharge fan. The implementation consists in stopping the rotation of the heat discharge fan from the time when a registration roller pair starts driving the sheet to the time when the leading edge of the sheet moves away from an outlet sensor.
Some problems arise when images with different resolutions are continuously printed on consecutive sheets. For example, assume that a first sheet is conveyed at the higher linear velocity for the lower resolution or is implemented by an ordinary sheet, and then a second sheet is conveyed at the lower linear velocity for the higher resolution or is implemented by a thin sheet. Then, high fixing temperature is assigned to the first sheet. Despite that lower target fixing temperature is assigned to the second sheet that immediately follows the first sheet, the temperature cannot be lowered to the target temperature and causes the second sheet to crease.
Also, assume that a first sheet is conveyed at the lower linear velocity or is implemented by a thin sheet, and then a second sheet is conveyed at the higher linear velocity or is implemented by a usual sheet. Despite that the higher target fixing temperature is assigned to the second sheet that immediately follows the first sheet, the temperature cannot be raised to the target temperature and fails to fully fix toner on the second sheet. In this manner, when different resolutions and different kinds of sheets are dealt with together, sheet feed occurs before the fixing temperature reaches the target temperature. This makes it impossible to fully protect sheets from creases and other defects.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 6-332330 and 2000-259045.
It is therefore an object of the present invention to provide an image forming apparatus capable of protecting a sheet from defective fixation and creases and enhancing image quality even when the kind of sheets to be used is varied or when a special sheet, for example, needing a particular fixing condition is selected.